Cell balancing

[Common]

Hello all,

As mentioned in a thread before this one I'm trying to diy a dive light.
Due to the amount of Leds it's going to be a 4s lion canister. (12 total 18650 cells, so 4s3p)
At this point I've decided which BMS to use but I'm a bit stuck on the balancing aspect. (BMS does not support it.)

Does a setup like this require (passive) balancing? All cells are new and the same model so any differences in capacity should be minimal I hope.
I'm trying to avoid having a small internal balancing board since I'm worried about the heat.

How do you guys make the battery setup? Internal balancing board, external balancing charger or no balancing at all?

Thank you.

Added some pictures of the internals as they are now in CAD. Totally overdesigned but that's the fun of it. 🤷‍♂️

 

[DIWdiver]

I think the general practice is to have a balancing charger, and a protection circuit in the pack.

Done well, an active balancing circuit is a bit bulky and expensive to put in every battery pack. Buy it once in your charger and you're done. Passive balancing isn't bad, but active can handle a much greater range of 'exceptions'.

The protection circuit is what absolutely must be in the pack if it's more than 1S. This just disconnects the output of the pack if something bad happens. The most basic ones will have overvoltage and undervoltage sensing for each 1S group. Better ones have overcurrent protection for the pack, with separate thresholds for charging and discharging. Some include temperature sensing as well.

Do you have a good welder? I haven't yet seen any other way to make sufficiently reliable and low-resistance connections.

 

[Common]

I think the general practice is to have a balancing charger, and a protection circuit in the pack.

Done well, an active balancing circuit is a bit bulky and expensive to put in every battery pack. Buy it once in your charger and you're done. Passive balancing isn't bad, but active can handle a much greater range of 'exceptions'.

The protection circuit is what absolutely must be in the pack if it's more than 1S. This just disconnects the output of the pack if something bad happens. The most basic ones will have overvoltage and undervoltage sensing for each 1S group. Better ones have overcurrent protection for the pack, with separate thresholds for charging and discharging. Some include temperature sensing as well.

Do you have a good welder? I haven't yet seen any other way to make sufficiently reliable and low-resistance connections.

Thank you for the response.
The protection will be a 4s PCM from Aliexpress. Atm I don't really have a way to test how reliable the board is though.
I've looked at the LiTechPower options as they are linked in the forums but they don't seem to be available here and in a small form factor they don't include balancing either.

With "balancing charger in the pack" do you mean a dedicated board (/ included in the internal BMS) or a balancing connector to connect to an external charger as is often used with LiPo's?
I know some seperate options exist such as in the first attached file but they all have very low balancing currents. 66mA for the shown pcb.

As for the welding I do plan to spot weld all battery/nickel strip connections. As for the cell per cell overcurrent I'm adding fuses soldered to the nickel strip if possible. I've tried to make a schematic in the second and third attached file. The fuses are only on one side of each cell of course. The other side is connected via welded nickel strip as is usual and connected to the series cell next in line.

I hope my thinking isn't too far of the safe way to make a pack?

 

[alpg88]

There are two ways to do it, i've done it before both ways, one way is to have a multiport plug, and use hobby charger in balance mode, another is to use a bms with balance/equilibrium function. but the way they work is a bit different from balance charger, most of them would discharge cells that have more voltage, by doing that bms makes heat, how much depends on a bms circuit. if you can get away with heat, or have ways of removing it, or insulating battery from it. it is easiest thing to do, you only have 2 wires to plug in. If any extra heat is unacceptable, use those cb mic plugs,, and hobby charger is the way to go, it is also kinda simpler way,( since you do not really need a bms, just some under voltage protection circuits,) those ports come in 3 ports to 8.

 

[Common]

There are two ways to do it, i've done it before both ways, one way is to have a multiport plug, and use hobby charger in balance mode, another is to use a bms with balance/equilibrium function. but the way they work is a bit different from balance charger, most of them would discharge cells that have more voltage, by doing that bms makes heat, how much depends on a bms circuit. if you can get away with heat, or have ways of removing it, or insulating battery from it. it is easiest thing to do, you only have 2 wires to plug in. If any extra heat is unacceptable, use those cb mic plugs,, and hobby charger is the way to go, it is also kinda simpler way,( since you do not really need a bms, just some under voltage protection circuits,) those ports come in 3 ports to 8.

In that case I think the hobby charger may be the way to go. Less problems with the design of the battery pack and those cb plugs seem really easy to implement.
Do you have any recommendations which hobby charger may fit best? Or is any 4s balanced charger acceptable?

 

[alpg88]

what charger, hm,.. here is the tricky part, as i found out the hard way.
there are 2 types of chargers, hobby chargers, that you can set to charge anything, in any way, those will work great, i use hitek x4, but any good quality hobby charger would do, that is usually around 50-70 bucks,
there are also simpler balance chargers, they are usually fixed current, no adjustments. 20-30 bucks, they work great with li poly packs, with no pcb, but when i tried to charge a pack that was cut off on low voltage, few of different balance chargers for li poly, would not "wake the battery up" to charge it, but it would charge a low battery no issues, as long as it was not cut off by pcb, hobby charger, otoh, "woke up" ever pack i build. i would not use those for a pack with pcb, use hobby charger

 

[Poppy]

I love this!

This is a prime example of the fact that there are people here at CPF who have specific knowledge and are willing to share that knowledge with others.